Data considerations for Nonnormal Capability Sixpack

To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.

The data should be continuous

Continuous data are measurements that may potentially take on any numeric value within a range of values along a continuous scale, including fractional or decimal values. Common examples include measurements such as length, weight, and temperature.

If you have attribute data, such as counts of defectives or defects, use Binomial Capability Analysis or Poisson Capability Analysis.

Collect enough data to obtain reliable estimates of process capability

Try to collect at least 100 total data points (subgroup size*number of subgroups), such as 25 subgroups of size 4, or 35 subgroups of size 3. If you do not collect a sufficient amount of data over a long enough period of time, the data may not accurately represent different sources of process variation and the estimates may not indicate the true capability of your process. Because the process data do not follow a normal distribution, try to collect subgroup sizes of at least 5 or more observations, if possible, to better ensure that the estimates of the control limits will be close to the true values.

The process must be stable and in control

If the current process is not stable, then the capability indices cannot be reliably used to assess the future, ongoing capability of the process. Use the control charts in the capability sixpack output to determine whether the process is stable and in control.

The data must follow the selected nonnormal distribution

If the selected distribution does not closely fit the data, the capability estimates will not be accurate. To determine which nonnormal distribution best fits your data, use Individual Distribution Identification.